work out the Mr values for the 2 compounds CaCO3 = 40+12+16+16 + 16 = 100. CaO is 40 + 16 = 56. This means 100g of CaCO3 would yield 56g of CaO. In the Q we are told we only have half that amount of CaCO3 = 50g, so you would halve 56 to ans = 28g